ARROW-2617: [Rust] Schema should contain fields not columns

[kszucs]

No description provided.

[codecov-io]

Codecov Report

Merging #2531 into master will increase coverage by 0.09%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #2531      +/-   ##
==========================================
+ Coverage   87.69%   87.78%   +0.09%     
==========================================
  Files         372       13     -359     
  Lines       57915     1392   -56523     
==========================================
- Hits        50788     1222   -49566     
+ Misses       7057      170    -6887     
+ Partials       70        0      -70

Impacted Files	Coverage Δ
rust/src/record_batch.rs	97.82% <100%> (-0.05%)	⬇️
rust/src/datatypes.rs	73.42% <76.92%> (ø)	⬆️
python/pyarrow/ipc.pxi
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
cpp/src/plasma/client.cc
cpp/src/arrow/io/test-common.h
cpp/src/arrow/python/io.cc
python/pyarrow/hdfs.py
cpp/src/parquet/column_scanner.cc
... and 351 more

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[kszucs]

Thanks Andy!